The news portal suggested that competition approval would be hard, given Sainsbury's and Asda's status as the second and third largest grocers in the United Kingdom respectively.

However things could be confirmed even sooner if the two sides are forced to accelerate their public disclosures. If it decides the new entity would have too much power in the market, the authority could block a deal or force the companies to sell stores.

"Sainsbury's confirms that it and Walmart Inc. are in advanced discussions regarding a combination of the Sainsbury's and Asda businesses", Sainsbury's said in a statement.

Sainsbury's Chief Executive Mike Coupe, who used to work for Asda, would lead it, the source said.

A combination between Sainsbury and Asda would create a grocery giant in the United Kingdom with over 30% market share. The Qatar Investment Authority is the company's largest shareholder, with a 22 percent stake.

The latest statistics show that Tesco has a 25 per cent grocery market share, while Sainsbury's has 13.8 per cent and Asda has 12.9 per cent. Asda did not respond to requests for comment.

Sky News, Bloomberg and the BBC have all independently reported the move, citing people with knowledge of the talks.

Walmart acquired Asda in 1999 when the focus of the company was on global growth.

It involved launching Argos concessions in Sainsbury's shops, encouraging shoppers to buy Argos products online, pick them up in-store and hopefully buy something in the supermarket along the way.

The competition watchdog is being urged to investigate the possible merger of Sainsbury's and Asda amid concerns over consumer choice.